The present invention relates to a lens, a manufacturing method thereof, and an improved optical pickup, and particularly to a lens capable of being accurately and efficiently manufactured, a manufacturing method thereof, and an optical pickup using the lens.
At present, the recording density of an optical disk has been increased as exemplified by a DVD (Digital Video Disk), and to realize the high density of an optical disk, it has been required to shorten the wavelength of a laser beam and to make a beam spot size small. In order to make a beam spot size small, it is necessary to increase the numerical aperture (hereinafter, referred to as xe2x80x9cNAxe2x80x9d) of a lens. The numerical aperture of a lens can be increased by making the diameter of the lens large, however, the size of the lens is restricted if the lens is assembled into a small-sized mechanical deck such as an optical pickup. In particular, it is difficult to manufacture a lens which is small in diameter having large radius of curvature. For this reason, to obtain a lens having a high NA, there has been used a so-called hologram integrally formed lens in which diffraction optics portions configured as blazed holograms are formed on a light incoming plane and a light outgoing plane of a geometrical optics portion configured as an aspherical lens. In addition, the hologram integrally formed lens is also used as an objective lens adapted to obtain two focal points for one lens.
A prior art lens manufacturing apparatus for manufacturing a hologram integrally formed lens is shown in FIGS. 1A and 1B. A lens manufacturing apparatus 1 shown in FIG. 1A includes an upper die 2, a lower die 3, and a core die 4. A hollow portion having a shape equivalent to that of a lens is formed between the upper die 2 and the lower die 3, and glass as a material for forming the lens is supplied to the hollow portion. The upper die 2 is movable in the direction shown by an arrow Y2 along the core die 4, to apply a pressure to glass. A hologram integrally formed lens is manufactured by executing the steps of; supplying glass into the hollow portion as shown in FIG. 1A, heating the upper die 2 and the lower die 3 to a glass formable temperature, moving the upper die 2 in the direction Y2 to press-form the glass, thereby transferring the shape of the die onto the glass. After cooling glass down, a hologram integrally formed lens is obtained.
Here, each of the upper and lower dies 2 and 3 must be accurately formed into a shape equivalent to that of a lens to be formed. FIGS. 2A and 2B show a state in which the die is shaped by machining. In addition, since the process of shaping the upper die 2 is the same as the process of shaping the lower die 3, only the latter process will be described with reference to FIGS. 2A and 2B, omitting the description of the upper die 2. Referring to FIG. 2A, the lower die 3 is composed of a base member 3b and a layer 3a to be machined. The layer 3a is formed of, for example, a film made from a noble metal such as platinum (Pt) or iridium (Ir). The reason for this is that since the melting point of glass is high, the layer 3a is required to be made from such a material which is not to be fusion-bonded to glass upon formation of the lens. The layer 3a is machined into a shape equivalent to that of a diffraction optics portion of a lens using a bite 5 made from diamond or the like. Then, as shown in FIG. 2B, after formation of the layer 3a into a specific shape, a protective layer 6 is formed on the surface of the layer 3a for preventing the diffraction optics portion from losing its shape.
The above method of machining the layer formed of the film made from a noble metal poor in machinability such as platinum (Pt) or iridium (Ir), however, has a problem in occurrence of heavy wear of the bite. To solve such a problem, it may be considered to replace the material of the layer to be machined, from a noble metal to a material exhibiting good machinability, for example, electroless-plated nickel, however, in this case, the electroless nickel plating has another problem that since the melting point of glass is high, the electroless-plated nickel layer on the surface of the die may be fusion-bonded to glass or severely consumed upon formation of the lens.
An object of the present invention is to provide a lens having a high numerical aperture which is capable of being accurately, efficiently manufactured at a low cost, a manufacturing method thereof, and an optical pickup using the lens.
To achieve the above object, according to a first aspect of the present invention, there is provided a lens including; a geometrical optics portion for converging light having been incident thereon from its light incoming plane, and two diffraction optics portions provided on the light incoming plane and a light outgoing plane of the geometrical optics portion, wherein the diffraction optics portions are made from a material different from that of the geometrical optics portion and joined each other. With this configuration, the diffraction optics portions, which are made from the material different from that of the geometrical optics portion, for example, a resin, are joined to the light incoming plane and the light outgoing plane of the geometrical optics portion, and accordingly it is possible to easily form the diffraction optics portions.
To achieve the above object, according to a second aspect of the present invention, there is provided a method of manufacturing a lens including a geometrical optics portion and two diffraction optics portions, including the steps of; inserting a material for forming the geometrical optics portion in a first die having a shape equivalent to that of the geometrical optics portion, press-forming the material by applying a pressure to the first die to form the geometrical optics portion, pouring a material for forming the diffraction optics portions in a second die having shapes equivalent to those of the diffraction optics portions, and inserting the geometrical optics portion having been formed by press-forming in the second die to join the geometrical optics portion to the diffraction optics portions. With this configuration, since a material for forming the diffraction optics portions, for example, a resin is joined to the light incoming plane and the light outgoing plane of the previously formed geometrical optics portion to form the diffraction optics portions, and accordingly, it is possible to easily form the diffraction optics portions on the geometrical optics portion.
To achieve the above object, according to a third aspect of the present invention, there is provided an optical pickup including; a light source for outputting a laser beam, a light splitting means for splitting the laser beam emitted from the light source into pieces, an objective lens for converging the laser beam from the light splitting means on a signal recording plane of an optical recording medium, and an optical detector for detecting a return laser beam reflected from the signal recording plane, wherein the objective lens includes a geometrical optics portion and two diffraction optics portions formed on the geometrical optics portion, the diffraction optics portions being made from a material different from that of the geometrical optical portion. With this configuration, the objective lens is configured such that the diffraction optics portions, which are made from the material different from that of the geometrical optics portion, are joined to the light incoming plane and the light outgoing plane of the geometrical optics portion, and accordingly it is possible to easily form the diffraction optics portions on the geometrical optics portion, and hence to provide a high performance optical pickup at a low cost by using a lens which has a high numerical aperture and a small spherical aberration and which can be efficiently manufactured at a low cost.